Mobile platforms are continually demanding better performance from their transducers, such as louder audio and better sound quality from their sound systems and better haptics performance. The transducers (e.g., including but not limited to speakers and haptics) in these systems can be damaged when they are pushed to their limits. One common failure mode for over-driven transducers (e.g. speakers/haptics) is thermal damage. As an example, for speakers, if the voice coil exceeds a maximum temperature, the glues that hold the voice coil together and connect it to the diaphragm can melt and cause irreparable damage. Speaker protection algorithms are commonly used to drive the speaker to its maximum volume while ensuring it does not exceed its rated limits.
FIG. 1 illustrates an example of an approach used in some thermal protection algorithms (for example, as illustrated in U.S. Patent Publication Numbers 2011/0194705 or US 2014/0169571). The audio input AIN is routed to a thermal limiter 101 that attenuates the audio input AIN based on the temperature of the voice coil in the speaker 102. As the coil temperature increases, the thermal limiter 101 applies an increasing amount of attenuation to ensure the voice coil does not exceed its thermal limit. A low frequency, low-level pilot tone 103 may be added to the input signal to aid in temperature detection as described in the next paragraph.
The input signal AIn comprising the pilot tone is routed to the amplifier 104 that drives the speaker. The amplifier 104 also provides a measurement of the speaker voltage, Vmon, and current, Imon. The Imon and Vmon signals may be used to determine a measured temperature, Tm, in temperature measure block 105 which may be used to control the amount of attenuation applied by the thermal limiter 101 to the input signal AIN.
FIG. 2 is an example implementation of the temperature measurement block 105. There are many possible variations on this algorithm for extracting a temperature from monitored signals across a transducer, and FIG. 2 illustrates only one possible implementation. First, a pilot tone extraction block 201 extracts the amplitude of the voltage and current signals at the pilot tone frequency. The pilot tone extraction block 201 may therefore be configured to perform, for example, narrow band filtering, a short Fourier transform, or heterodyning. The output of the pilot tone extraction block 201 may then be optionally low pass filtered by filter 202 to reduce noise, although the processes performed by the pilot tone extraction block 201 may involve some inherent low pass filtering. Next, the pilot tone voltage level VPT is divided by the pilot tone current level IPT to yield an estimate, Re, of the direct current (DC) impedance of the transducer. The pilot tone may be set at a frequency where the corresponding transducer impedance is a close approximation of its DC resistance. Again, the estimate, Re, may be optionally low pass filtered by filter 203 to reduce noise. Finally, the calculated resistance R is converted to temperature, Tm, by the resistance-to-temperature conversion block 204, using the temperature coefficient associated with the transducer. The accuracy of the measured temperature Tm may be improved by calibrating the system at a known temperature.
As mentioned above, the pilot tone may be chosen at the frequency to ensure the calculation of the DC resistance is sufficiently accurate. In addition, the pilot tone may be selected to be as inaudible as possible, since it may add distortion to the outgoing audio. Typically, these constraints lead the designer to use a low frequency (e.g., <100 Hz), low level (e.g., <30 dB) pilot tone for speakers.
Market and industry trends are making it more difficult to design thermal protection algorithms that provide high quality output while protecting transducer. These difficulties may be understood by visualizing the temperature protection algorithm as a linearized control loop, as illustrated in FIG. 3. The plant 301 is the thermal model of the speaker or transducer. The input to the plant 301 is the thermal power Pth that heats the transducer and the output of the plant 301 is temperature of the voice coil T. The temperature, T, may not be measured directly; it is instead estimated, for example as described in FIG. 2. The methods for estimating the temperature involve some form of low pass filtering to filter out measurement noise, and to extract the voltage and current amplitude at the pilot tone. This filtering is illustrated by the low pass filter 302. Some measurement noise may originate from the Imon and Vmon Analog-to-Digital Converter (ADC) hardware as well as from numerical noise from the fixed-point implementation of the impedance estimation algorithm. This measurement noise may be included in the input to the temperature estimation, for example, added to the input of the low pass filter 302. The thermal limiter 303 compares this measured temperature Tm to the temperature limit associated with the speaker or transducer, and applies a scaling factor, K, to determine a gain, G, applied to the audio input signal AIN. This determined gain will adjust how much thermal power, Pth, is delivered to the speaker along with other gains intrinsic to the system, illustrated in the system gain block 304. In this linearized form, the control scheme is a proportional control loop with some low pass filtering as part of the temperature estimation on the feedback path.
The design of the low pass filter used to estimate the temperature, in FIG. 3, may be critical to achieving good performance from the control loop. Too much or too little filtering may yield unacceptable results. For example, FIG. 4a illustrates the results when there is too little filtering applied (i.e., cutoff frequency is too high). FIG. 4a shows a simulation of the thermal protection closed loop response when a full-scale heating tone (e.g., 4 kHz) is applied with a thermal limit of 100° C. The top plot shows the applied gain from the thermal limiter 303, and the bottom plot shows the actual and estimated coil temperature. From the bottom plot, it can be seen that the estimated temperature in this example is very noisy. This noise causes excessive gain fluctuation from the thermal limiter 303, as shown in the top plot. This excessive gain fluctuation may result in an unacceptable level of audible distortion.
FIG. 4b illustrates an example of the results when there is too much filtering applied (i.e., cutoff frequency is too low). In this case, the estimated temperature is smooth but has too much delay, as seen in the bottom plot of FIG. 4b. Excessive delay in the feedback path may cause a degradation in phase margin or, equivalently, a reduction in damping ratio (see Nise, Norman S., “Control Systems Engineering” Menlo Park: Addison-Wesley Publishing Company, 1995 pp 594-596). Such degradation/reduction can cause overshoot and excessive ringing in the response, as seen in FIG. 4b. Such a response may be unacceptable because the overshoot goes well above the temperature limit, and ringing also creates an undesirable pumping effect in the audio levels. A well-designed thermal limiter 303 may therefore have a balance between too much and too little low pass filtering. However, market and industry demands are pushing toward an increase in the measurement noise and/or a reduction in phase margin (stability).
Lower amplitudes are being requested to reduce the amount of direct or intermodulation distortion introduced by the pilot tone frequency. A lower amplitude pilot level also increases the usable excursion range of transducer, for example, leading to a potential increase in sound pressure level (SPL) or acceleration for a haptic transducer. However, lower amplitude pilot tones decrease the signal-to-noise ratio (SNR) of the measured temperature Tm. Such an SNR decrease may cause an effective increase in the Measurement Noise illustrated in FIG. 3, thereby reducing the accuracy of the measured temperature Tm.
A demand for louder output levels and more dynamic range have driven a new generation of boosted amplifiers with increased voltage outputs. Such increased voltage outputs correlate to the speaker heating up faster, making the system more sensitive to delays from the low pass filter 302. The reason for the higher sensitivity to delays is that the higher system gain 304 in the forward path causes a reduction in phase margin.
There is also a demand for smaller integrated circuits. The size and cost of Analog-to-Digital converters (ADCs) can be reduced by reducing the requirements on their resolution. However, a lower resolution ADC will increase the measurement noise on the temperature estimate Tm.